More than 3 million US adults may have inflammatory bowel disease, according to a new government estimate. That’s nearly triple the number of some previous estimates, the researchers said.

The new estimate is based on a national survey conducted by researchers at the Centres for Disease Control and Prevention (CDC). Survey respondents were asked whether a doctor or other health professional had ever told them that they had either Crohn’s disease or ulcerative colitis, which are the two types of inflammatory bowel disease (IBD). Based on the responses, the researchers estimated that 1.3 percent of US adults, or 3.1 million Americans, have IBD.

People with IBD have chronic inflammation in the gastrointestinal tract. Patients often have abdominal pain, cramping, fatigue and diarrhoea. They may also have a poor quality of life, as they often have complications and need to be hospitalized or undergo surgery, the report said.

“According to this report, the prevalence of IBD is much higher than previously estimated,” said Dr Siddharth Singh, a gastroenterologist and clinical assistant professor of medicine at the University of California, San Diego School of Medicine.

Knowing the true rate of IBD is important because that knowledge will help health care providers offer better “strategies for high-value care” to patients with the condition, Singh told Live Science. It will also help researchers understand the impact of this condition on the health care system, he said.

The report additionally found that IBD is more common in some groups, including adults ages 45 and older, Hispanics, non-Hispanic whites, and adults with less than a high school level of education.

“For a disease traditionally thought to affect young adults, it is surprising to see a high prevalence of [IBD] in older adults,” Singh said. The report found that 1.5 percent of adults ages 45 to 64, and 1.7 percent of adults ages 65 and older said they had been diagnosed with IBD.

In the new report, the researchers looked at data gathered in 2015 during the CDC’s annual National Health Interview Survey. In this survey, researchers conduct in-person interviews with participants from across the US about a broad range of health topics.

Previous estimates of IBD prevalence in the US have come from surveys done in limited geographic areas, or from health care claims data. For example, a study published in 2013 used claims data from 12 million people and estimated that 1.2 million US adults had IBD. A 2007 study, based on the residents of one county in Minnesota, estimated that, nationally, 1.1 million people had the disease.

However, Singh said that the new report may overestimate the condition’s prevalence. The symptoms often seen in patients with IBD may also occur in other, more common gastrointestinal diseases, such as irritable bowel syndrome, he said. A diagnosis of IBD needs to be confirmed with an exam done using an endoscope (a device that has a small camera mounted on flexible tube, which doctors use to view the inside of the gastrointestinal tract), Singh said.

“Some individuals may misinterpret their symptoms” and think that they have IBD when they may actually have irritable bowel syndrome, he added.

The authors of the new report, led by James Dahlhamer of the CDC’s National Centre for Health Statistics, also noted that a limitation of the new estimate was that it relied on people’s self-reports of being diagnosed with the condition.

There is no known single cause of IBD. “IBD is complex disease, and is caused by an interplay of several genetic and environmental factors, such as diet and [the] intestinal micro biome and our immune system,” Singh said.

The increasing rate of the condition in the US could be linked with several factors, including a change in people’s diets that involves eating more packaged food or fast food, and increasing fat and sugar consumption, he said. But the increased use of antibiotics, dietary chemicals and the increasing prevalence of obesity may all also play a role, he said.

Most patients with IBD tend to have normal life spans, but “their quality of life may be significantly impacted,” he said.

The new report was published Oct. 28 in the CDC’s Morbidity and Mortality Weekly Report.